Drilling apparatus

ABSTRACT

A drilling apparatus for drilling holes of predetermined depth in a workpiece including a drill head assembly disposed for angular orientation with respect to the workpiece. The assembly comprises a drill mechanism including a powered drill bit with controls for automatically controlling rotational speed of the bit. The drill mechanism is moved axially into and out of engagement with the workpiece by a drill control mechanism including a fluid motor unit with controls for automatically controlling the feed rate of the bit, such as rapid forward feed, intermediate forward feed, reduced constant cutting feed and rapid retraction feed, in each cycle, and with the cycle length and duration automatically determined in accordance with load forces, such as chip load or the like, on the bit.

United States Patent Hayes [451 Jan. 25, 1972 [54] DRILLING APPARATUS211 App]. No.: 843,673

Primary Examiner- Francis SQ Husar Attorney-Teare, Teare & Sammon [57]ABSTRACT A drilling apparatus for drilling holes of predetermined depthin a workpiece including a drill head assembly disposed for angularorientation with respect to the workpiece. The assembly comprises adrill mechanism including a powered drill bit with controls forautomatically controlling rotational speed of the bit. The drillmechanism is moved axially into and out of engagemem with the workpieceby a drill comm. mechanism 58 n d 408/] 1 l7 eluding a fluid motor unitwith controls for automatically controlling the feed rate of the bit,such as rapid forward feed, intermediate forward feed, reduced constantcutting feed and [56] References Cited ra id retraction feed, in each ccle, and with the c cle len th P y y 8 UNITED STATES PATENTS andduration automatically determined in accordance with load forces, suchas chip load or the like, on the bit, 3,418,549 12/1968 Emerson et al...77/32.7 UX 3,259,023 7/1966 Rieger et al..,.. ..77/32.7 X 25 Claims,19 Drawing Figures 2,418,387 4/1947 Zarobsky 77/32.7 X 2,260,327 10/1941McKee ..77/32 4 CARRIAGE SWlTCH I5 canmee cm roLLoweR PREMPCT pusmzocwsss '2: g rmesn I50 Fomnao SWITCH I4A TRIPS BY CARRIMI a COOLANTSWITCH 5A COOLANTOFF ARM '9 FAS T RETRACT AC TACH CHIP FORWARD a DROPSOUT BU|LD up SLOW COOLANT PATENTED M2 312 3,637,318

SHEET OlUF 12 J w 2 l I FIG.I

INVENTOR. RICHARD H. HAYES BY Maui/M ATTORNEYS PATENTED JAN25 1972 SHEET03 0F 12 5 NF wk F M I 3 L H mm A 5 |Yo 1G I IL. mm: mm B m mg m 3 X A 3g 3% N3 v a m fi 3 3 Nu Ill 8 E A? a g 2 m2 1 NVENTOR.

RICHARD H. HAYES ATTORNEYS PATENTED JAH25 I972 SHEET OSUF 12 INVENTOR.

H. HAYES RICHARD BY 76044 za/wfm ATTORNEYS FIG.|O

INVENTOR. RlCHARD H. HAYES ATTORNEYS PATENTEDJANIZSIQYZ 3.637318 SHEET07 0F 12 Fl 6; I l

INVENTOR. RICHARD H. HAYES ZML MJM ATTO R N EYS PATENTED masmz 3.637.318SHEET 08 0F 12 v INVENTOR RICHARD H. HAYES BY 724/21, 7am {W ATTORNEYSPATENTED JAN251972 SHEET IOUF 12 J r l i FIG.I4

INVENTOR. RICHARD H. HAYES ATTORNEYS PATENTED JAN25 1972 SHEET 11 0F 12'62 vou-mss COMPARISON- AMP PULSE 50R PHASE SHIFT GENERA 2 30 I CIRCUITI I 9 8| FLUID RESERVOIR 3:9 54 00 f 3 2 q fgggg 38R e r 3v INVENTOR.SENSOR 4 CIRCUIT RICHARD H. HAYES ATTORNEYS PATENTED Jmsmz 3.637.318

SHEET 12 HF 12 CARRIAGE swncn I5 CARRIAGE cm aga FOLLOWER f 'mxJ PUSHEDCLOSES FEED FINGEI-LISESO a FORWARD SWITCH |4A TRI BY CARRIAGE I aCOOLANT SWITCH 5A ICOOLANTOFF ARM '9 FAST DC RETRACT Ac TACH CHIPFORWARD TACH a DROPS OUT BU|LD p I SLOW OPERATES COOLANT Lslw qf t,| +l

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I LSl- F:%F F g Y LS2 C x5 LSI J LS2 START V A D INVENTOR.

RICHARD H. HAYES Zea/:2, M r/M ATTORNEYS BACKGROUND OF THE INVENTION Thepresent invention relates to drilling apparatus, and more particularlyto an apparatus for automatically drilling small diameter and relativelydeep holes in a workpiece, such as in the venting of metallic tire moldsor the like.

Heretofore, the drilling of holes having a diameter of the order ofone-sixteenth of an inch in a metallic workpiece has been an appreciableproblem, particularly where such holes are relatively deep, such as 6 to8 inches in depth. In drilling operation, the chip buildup causes thedrill bit to bind in the hole resulting in breakage of the drill bit.Consequently, the drill bit must be repeatedly withdrawn at the propertime, the chips removed and the drill reinserted. However, prematurewithdrawal of the drill bit greatly increases the drilling time, whereaslate withdrawal results in breaking of the drill bit. These and otherproblems existed even when the drilling was performed by skilledoperators. The work was dangerous for when the drill would break, smallfragments would be thrown off causing liability of injury to theoperator. Usually, the operator cannot reduce the hazard by slowing downthe rate of rotation when a break occurs, because the breakage andconsequent explosion of the particles occur before the operator has anopportunity to reduce the drill speed or stop the power. In cases whereholes have been drilled from the inside of a workpiece, such as a tiremold or the like, the operator has stood in front of the workpiece andpulled the drill toward himself. In such operations, accidents haveoccurred when the drill unexpectedly penetrated through the mold withconsequent injury to the operator.

Heretofore, still further problems have existed with manual drillingoperations. One such problem generally occurred when a man standingoutside the mold experienced difficulty in backing the drill out of thehole or the like. importantly, it was often difficult for the operatorto drill a straight or substantially linear hole. About 200 pounds ofpressure is usually required to be exerted on the drill bit in order toforce the same into'the material of a tire mold. In order to achievesuch pressure, the operator has generally had to lean against a supportor against the workpiece. Such techniques, however, have made itextremely difficult to maintain proper accuracy in hole alignment. As aresult, the drilled holes have often had a bend or curve rather thanbeing straight or linear throughout their length, as desired. Moreover,the difficulties and attempts to avoid such problems in drillingstraight holes have oftentimes increased the likelihood of drillbreakage.

Heretofore, automatic drilling machines have been provided for drillingholes in various metallic materials. However, such machines have notbeen satisfactory for drilling smalldiameter and relatively deep holesin such materials. Generally, either the machines did not retract duringthe drilling of a hole for chip removal and/or lubrication, or thewithdrawal of the drill bit was on a predetermined basis. However, wherethe retraction was predetermined, it was generally not satisfactorysince it was pursuant either to a distance or time cycle. In such cases,the drill bit would be withdrawn when it had drilled a predeterminednumber of inches or when it had been drilling for a predetermined time.Since it is generally impossible to predeterrnine the extent of drillingbefore the drill bit must be retracted, such machines had to be eitherset for too slow a speed, and too frequent withdrawal, or they resultedin excessive drill breakage. Not only is there variation from hole tohole for the distances at which the drill must be removed, but there isalso a variation in the number of times the drill bit must be removed.The distance a drill bit can travel before removal depends on manyfactors. These factors include, for example, the nature of the metallicmaterial at the hole location, lubrication of the drill bit, sharpnessof the drill bit, etc. As a result, the drill bit, in the automaticdrilling machines, had to be repeatedly withdrawn prematurely, therebyincreasing the drilling time with consequent wasting of lubricant and/orcooling material, or the drill bit would not force load variations bewithdrawn soon enough, thereby causing excessive breakage. As aconsequence, the skilled operator, even with the aforementioned andother difficulties, still remained the most reliable and expeditiousmethod for drilling relatively deep, small-diameter holes.

SUMMARY OFTHE INVENTION A drilling apparatus for drilling one or moreholes in a workpiece by one or more random cycles of indeterminateduration or length comprising, a frame for mounting said workpiece andfor supporting a drill head assembly for angular orientation withrespect to said workpiece. The assembly includes a drill mechanismhaving a powered drill bit and control means for automaticallycontrolling the rotational speed of said bit. A drill control mechanismis attached to said drill mechanism and includes a fluid motor means formoving said drill mechanism relatively to said drill control mechanismand for moving said drill bit axially into and out of engagement withsaid workpiece. A control system coacts with said drill controlmechanism and said fluid motor means for automatically and sequentiallycontrolling the feed rate of said bit, such as by a rapid forwardfeed-intermediate forward feedreduced constant cutting feed-and rapidretraction feed, for a given cycle of operation in response to forceload variations on said drill bit.

The apparatus further includes a bracket assembly for pivotally mountingthe drill head'assembly with respect to said frame for selectivelyangularly orienting the drill bit with respect to said workpiece.Powered means are mounted on the frame for vertically raisingandlowering said drill head assembly with respect to said workpiece, andtablelike means are mounted on said frame for rotatably supporting saidworkpiece with respect to said drill head assembly. A drill bitservicing means coacts with the drill head assembly to cool and/orlubricate said drill bit and/or to remove chips or the like therefromduring axial movement thereof with respect to said workpiece.

By the foregoing arrangement, the drill bit is caused to travel axiallyat a maximum rate until just prior to contacting the workpiece, at whichoint it automatically slows to a reduced intermediate or preimpact rate.Shortly after impact with the workpiece, the bit slows automatically toa substantially constant axial feed rate for cutting the workpiece. Thebit continues at this substantially constant cutting rate until areimparted thereto, such as due to chip buildup, wear or the like,whereupon, the drill bit is automatically retracted, cleaned, cooled orlubricated, and then recycled. By this arrangement, the depth of travelof the drill bit prior to each retraction thereof is maintained at amaximum while the drilling time is maintained at a minimum with asubstantially constant cutting rate being maintained throughout thedrilling operation. Accordingly, drill breakage is greatly reduced, andin most cases, eliminated with the capability to drill holes morerapidly and straighter than can be achieved by heretofore knownapparatus, yet without consequent danger to the operator or others inthe work area.

In addition, the drill head assembly is of a small and com-' pact, yetrugged construction with long stroke capacity for operation in confinedareas-The assembly is of a lightweight and highly sensitive constructionwith extremely long-life features, minimum wear and maintenance ease forhigh-production use. The assembly is of a self-contained constructioncapable of handling very small diameter drills, such as 1/32- inch to3/1 6-inch diameter, to substantial depths, such as to 8 inches, withsubstantially automatically controlled feed rates. Also, the assemblyadjusts to torque requirements of selected drill size and materials andadjusts to pressure requirements, and further'adjustsautomaticallyto'the desired cutting depth.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation view of thedrilling apparatus mounting a drill head assembly in accordance with theinvention;

FIG. 2 is an enlarged fragmentary front elevation view looking in thedirection of the line 2-2 of FIG. 1;

FIG. 3 is a fragmentary top plan view of the drill head assembly removedfrom the apparatus of FIG. 1;

FIG. 4 is an end view looking from the left-hand side of FIG.

FIG. 5 is a fragmentary side elevation view of the drill head assemblyof FIG. 3;

FIG. 6 is a horizontal section view taken along the line 6-6 of FIG. 5with parts cut away for purposes of clarity;

FIG. 7 is a vertical section view taken along the line 7-7 of FIG. 3,with parts removed for purposes of clarity;

FIG. 8 is a fragmentary top plan view showing the interior of the drillhead control mechanism with parts removed for purposes of clarity;

FIG. 9 is a vertical section view FIG. 8;

FIG. 10 is an enlarged fragmentary vertical section view taken along theline 10-10 of FIG. 5;

FIG. 11 is an enlarged fragmentary horizontal taken along the line 11-11 of FIG. 5;

FIG. 12 is a vertical section view taken along the line 12- 12 ofFIG.l1;

FIG. 13 is a diagrammatic illustration showing a portion of a typicalelectrical control circuit for use in the invention;

FIG. 14 is a diagrammatic illustration showing another portion of-theelectrical circuit for use in the invention;

FIG. 15 is a diagrammatic illustration of a further portion of theelectrical control circuit for use in the invention;

FIG. 16 is a schematic diagram of a typical feedback system which may beused in the invention;

FIG. 17 is a schematic diagram of a sensing system for use in theinvention;

FIG. 18 is a schematic diagram showing the sequence of operation of thedrilling apparatus of the invention; and

FIG. 19 is a graphic illustration of a typical cycling operation of thedrilling apparatus of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, and with referenceto FIG. 1 of the drawings, there is illustrated the drilling apparatus 2of the invention for drilling one or more holes in a workpiece W, suchas in the venting of a metal tire mold or the like. As shown, theapparatus 2 includes a frame 4 having a base 6, an upstanding pedestal 8and a cantilevered crossarm 10 for supporting a drill head assembly 20above the workpiece. The base 6 mounts a rotatable support table 14 sothat the workpiece can be selectively rotated about a vertical axis withrespect to the drill head assembly 20. The drill head assembly 20 ismounted to depend from the crossarm 10 by a support column 16 (FIG. 2)so that the assembly can be selectively raised and lowered in a verticaldirection toward and away from the workpiece. The assembly 20 is mountedon the column 16 by a bracket assembly 18 so that the drill head 20thereof can be selectively tilted about a horizontal axis (shown by thearrows) for drilling holes angularly in the workpiece.

- The drill head assembly 20 generally comprises a drill mechanism 22and a drill control mechanism 24 supported, as a unit, from the bracketassembly 18. The mechanism 22 includes a rotatable speed controlleddrill bit 23 and is mounted below and adapted for reciprocal axialmovement with respect to the drill control mechanism 24 for selectivelycontrolled feed axially toward and away from the workpiece W. The drillcontrol mechanism 24 includes mechanical and electromechanical controlcomponents for automatically controlling rotational (drill speed) and/oraxial (drill feed rate) movements of the drill bit 23 with respect tothe workpiece W in response to force load variations, such as chip loador the like, on the bit, as will be described hereinafter.

In operation, for example, the drill mechanism 22 for a given drillingcycle has a controlled sequential feed rate, such as a rapid forwardfeed-intermediate forward feed-reduced cutting feed-and, rapidretraction feed, with such cycle autotaken along the line 9-9 of sectionview matically repeated for the next successive drilling operation. Theterm cycle, as employed herein, shall mean the sequential steps forcompletion of the drilling operation for a hole to be drilled to a givendepth.

By the foregoing arrangement, a plurality of small-diameter holes may beprecisely and efficiently drilled to a given depth in a workpiece. Morespecifically, in the invention a given hole depth may .be achieved by apredetermined incremental removal of stock so that as the drill bit 23penetrates into the workpiece the amount of cutting depth per cycledecreases (i.e., stock to be removed) while the noncutting depthincreases. Accordingly, in the invention the actual cutting feed (i.e.,reduced forward feed) is maintained at a substantially constant ratewhile certain of the noncutting feeds (i.e., rapid forward andretractive feeds) are maintained at a maximum rate for a given cycle.

In the embodiment shown, the bracket assembly 18 includes a generallyU-shaped bracket 30 (FIG. 2) having a pair of oppositely disposedsidearrns 32 and 34 connected together by a baseplate 35 which, in turn,may be connected to the support column 16, as by weldments or the like.The drill head assembly 20 is preferably mounted between the sidearrns32 and 34 for pivotal movement about a horizontal pivot axis 33 (FIG. 1)by a pair of oppositely disposed sideplates 36 and 38 which may bejoumaled for rotation, as at 40, to the respective sidearms 32 and 34.The assembly 20, in turn, may be removably attached to the sideplates 36and 38 by means of keyways 42. Pivotal movement may be imparted to theassembly 20 by means of a segment gear 44 fixedly attached to one of theplates, such as 36, so as to be disposed for meshing engagement with aworm gear 46 (FIG. 1) rotatably mounted on the column 16 by suitablebrackets 47. The gear 46 may be driven, such as by a manual handle 48,or a suitable motive power (not shown), for selectively angularlyorienting the drill bit 23 with respect to the workpiece.

As best seen in FIGS. 3 to 7, the drill mechanism 22 includes afluid-actuated, such as pneumatic, motor unit 50 having a chuck 52 forremovably mounting the drill bit 23. Rotational movement may be impartedto the motor unit 50 via conduits 53 from a source of pressurized fluid,such as an air compressor or the like, (not shown). The motor 50 and thecompressor therefor may be controlled by solenoids 321 and 335 (FIG. 15)for regulating the rotational cutting speed of the bit 23. For example,a low and high speed can be imparted to the drill bit, such as 10,000r.p.m. and 20,000 r.p.m., as desired.

As best seen in FIGS. 5, 11 and 12, the rotational speed of the motorunit 50 may be sensed by a sensing unit 54, such as an AC tachometer toprovide a signal for retraction of the drill bit 23 upon excessive loadforce thereon. The unit includes a housing 55 attached to the motor unit50 and is preferably in the form of a generator having a rotor 56, suchas magnet or the like, attached to one end of the drill bit, as at 57.The rotor 56 is spaced from the stator 58 which includes a field coil 59disposed around a spool 60 attached interiorly of the housing 55 so thatupon rotation of the bit 23 and henceithe rotor 56, the magnetic fieldgenerated produces an electrical voltage output which correspondsproportionately to therotational speed of the bit. By this arrangement,when the outputvoltage from the generator drops below a predeterminedvalue, an output signal is produced for automatically retracting the bit23, as will be described hereinafter.

In the form shown, the sensing unit 54 may be provided with apressure-responsive overload device 61 to cause retraction of the drillbit 23 upon wear or the like thereof. Such device may include a mountingbracket 62 attached to the underside of the drill control mechanism 24(FIG. 1). The bracket is arranged to threadably receive an adjustmentscrew 63 which, in turn, mounts a ball plunger 64 which may bespring-biased for engagement with the confronting face 65 of a recessedportion 66 provided in the housing 55. By this arrangement, the devicemay be threadably preset so that when the back pressure on the drill bit23 exceeds a predetermined pressure, the ball plunger 64 is depressed toactuate a suitable switch (not shown) in the circuit to automaticallyretract the bit for 'sharpening and/or replacement thereof.

The drill control mechanism 24 includes a polygonal, hollow housing 67having opposed end 68 and side 69 walls (FIG. 6) with a removable topplace 70 (FIG. 7) attached, as a unit, to the drill mechanism 22. Thesidewalls 69 include the keyways 42 to provide the pivotal connectionwith the bracket assembly 18. The housing 67 is preferably of an openbottom construction for mounting a slide carriage 71 which supportstherebelow the drill mechanism 22 for reciprocal movement of the samewith respect to the mechanism 24.

In the invention, the slide carriage 71 includes a pair of parallel,hollow guide rods 72 and 73 which extend between and are fixedlyattached to the end walls 68 (FIGS. 8 and 9) of the housing 67. The rods72 and 73 each slidably receive in telescopic relation therein sliderods 74 and 75 which extend forwardly through apertures, as at 76, inthe forwardmost end wall 68. A support plate 77 (FIG. 7) is attached atone end to a pair of rear blocklike yokes 78 which are slidably mountedon the exterior of the guide rods 72 and 73 and at the other end to acommon transverse yoke member 79 which extends between and is fixedlyattached adjacent the distal ends of the respective slide rods 74 and 75for reciprocal telescopic movement upon axial movement of the rods 74and 75 within the rods 72 and 73. The drill mechanism 22, in turn, isdetachably mounted in underslung relation below the support plate 77 viasuitable fasteners, as at 80, and by the aforesaid bracket 62, as seenin FIG. 7.

In the invention, reciprocal movement is preferably imparted to theslide carriage 71 by means of a fluid motor unit 81 (FIG. 7) disposedgenerally interiorly of the housing 67 above the support plate 77. Theunit 81 is preferably of the double-acting type including a cylinder 82fixedly attached between the end walls 68 and a piston rod 83 whichextends through an aperture, as at 84, in the forwardmost end wall 68for detachable connection via suitable fasteners 85 to the front yokemember 79. Fluid pressure may be introduced into the cylinder 82 viasuitable conduits 82a and 82b from a servo valve mechanism 86 (FIGS. 2and 5) which may be mounted on the exterior of the rearwardmost end wall68 for controlling actuation of the motor unit 81 and hence, thereciprocal movement of the drill mechanism 22 with respect to theworkpiece W. The servo valve mechanism 86 may be of the two-directionaltype for varying speed in response to electrical signals and may be ofthe type, such as made by Champion Manufacturing Company, Cadillac P/NFCIO624A. In the form shown, the solenoid mechanism 86 includes a spoolvalve (not shown) which is actuated in response to energization of asolenoid coil 90 (shown diagrammatically in FIG. 13). A suitable pump(not shown) may be provided for introducing fluid under pressure to themechanism 86 and for establishing fluid exhaust therefrom, as known inthe art.

In a preferred form, when the solenoid 90 is energized, the spool valvewill move to a full open position to cause the piston 83 to move forwardat a high speed, such as 500 inches per minute. For the remaining twoforward speeds, the spool valve is selectively partially closed. Thus, arelatively lower energization causes the spool valve to partially closeso as to reduce the forward speed of the piston 83 to the preimpact orintermediate speed, such as inches per minute. A further reduction inenergization of the solenoid coil 90 acts to cause the spool valve tocorrespondingly fluctuate in small movement. Since fluctuation inenergization of the solenoid coil 90 may occur more readily thancorresponding changes can occur in the fluid flow rate, the fluid flowto the motor unit 81 can be maintained at a constant desired low rate,such as 6 inches per minute, for cutting the workpiece. A reverseenergization of the solenoid coil 90 will cause the spool valve to moveinto its neutral position which, in this case, provides a full fluidflow in a reverse direction at a high speed so that the piston 83 andhence, the direction, such as at 500 inches per minute.

A service means 130 may be mounted on the carriage 71 adjacent the drillbit 23 so that a coolant and/or lubricant, such as a mixture of air andoil or the like, may be blown onto the bit at selective portions of thedrilling cycles. In the form shown, the service means includes an outeryoke 129 fixedly attached adjacent the distal ends of the slide rods 74and 75, as best seen in FIGS. 3 and 5. The yoke mounts a sleeve 131which supports the forward end of the bit 23 and includes an inletnozzle 132 which supplies the coolant and/or lubricant via a conduit 133from a suitable supply source (not shown) through a hole (not shown) inthe sleeve 131 to the drill bit 23. Flow through the nozzle 132 may becontrolled by a suitable solenoid-actuated valve (not shown) which inturn may be controlled by a solenoid 352 (FIG. 15).

In the invention, a drill bit guide device including a second yoke 135(FIGS. 3 and 5) may be mounted on the slide rods 74 and 75 and generallyintermediate the forward yoke 129 and the carriage yoke 79. By thisarrangement, the yoke 135 acts to further guide and align the slide rods74 and 75 and provides an additional support for the drill bit 23.

In accordance with the invention, the drill control mechanism 24includes a triggering system in the form of mechanical-electricalcomponents for the selective transmission of electrical signals to thecontrol system for sensing and controlling the axial position and/orspeed of axial movement of the drill mechanism 22 and for controllingand sensing the cutting action of the drill bit 23.

As best seen in FIG. 9, the triggering system includes a forward triggerdevice 136 which may be mounted interiorly on one of the end walls 68 ofthe housing 67. In the form shown, the device may include a limit switch14a having a fixed contact 137 and a movable contact 138 which may beelectrically connected to the control circuit. The switch is preferablymounted in the path of travel of the carriage 71 so that a cam 139carried thereby actuates contacts 137 and 138 when the carriage is inits rearwardmost retracted position. By this arrangement, the closing ofthe switch sends a surge of current to actuate fast forward movement ofthe carriage 71 and hence, the drill mechanism 22 when the carriage isin the retracted position, as will be described hereinafter.

In the form shown, the cam 139 constitutes an extension of another sliderod 140 (FIG. 9) mounted for reciprocal movement within the housing 67.The rod 140 preferably has a length i to extend over the trigger device136 in its fully retracted position thereof. The rod 140 extends at itsother end through an aperture 141 in the forward end wall 68 and isfixedly connected adjacent its distal end to the carriage yoke 79 forreciprocal movement upon axial movement of the carriage 71. At one end,the rod mounts a follower device 142 including an upstanding generallyL-shaped (FIG. 8) cross-plate 143 which carries a horizontally extendingfinger element 144. The follower device 142 is fixedly connected at oneend to a drive mechanism 145 and is disposed with the finger element 144for coacting pushing engagement with respect to a position indicator 146upon actuation of the drive mechanism 145.

In the form shown, the position indicator device 146 is slidably mountedon another rod 147 which is disposed above (FIG. 9) the slide rod 140and may be fixedly connected between the respective end walls 68 of thehousing 67. Such device is preferably of an inverted L-shapedconstruction (FIG. 8) adapted to be axially slid upon the rod 147 uponabutting engagement by the follower device 142. The device 146 includesa movable contact element 148 adapted to be engaged by the fingerelement 144 so as to close a limit switch 15a for actuating the controlcircuit for selective movement of the carriage 71 and hence, the drillmechanism 22. The switch 15a may include a fixed contact 149 and amovable contact 150 connected to a depending angularly oriented contactfinger 152 (FIG. 9) which rides along a conductor bar 153 carriage 71travel in a reverse mounted on a support plate 154 which is fixedlyconnected to and extends between the respective end walls 68 of thehousing 67. The bar 153 may be connected by suitable conductors clude aswitching means 170 (not shown) with a portion of the control circuitwhich initiates thepreimpact or intermediate forward speed of thecarriage the preimpact The movable contact 150 may be in the form of aleaf spring having a roller-type cam 155 mounted on the end thereof. Thecam 155 is preferably disposed in the path of the finger element 144 ofthe follower device 142 so that as the finger element moves internallyof the position indicator 146, the contact of the finger with the camcloses the circuit which initiates the preimpact or intermediate slowspeed of the drill mechanism 22. Preferably, the length of the fingerelement 144 is determined so that when the switch a is closed by thecontact between the finger 144 and the cam 155, an approximately 76-inchspace exists between the inner surface, as at 156, of the followerdevice 142 and the mating surface, as at 157, of the position indicatordevice 146.

In the invention, the position indicator device 146 provides a drive forthe drive mechanism 145. As shown, the device includes a connecting pin158 which is fixedly attached to an endless drive element 159, such as alink chain or the like, which may be trained around (FIG. 8) freelyrotatable sprockets 160, (only one shown), which may be journaled withinthe housing 67. The element 159 may be connected through suitablegearing, as at 161, to the input end of a sensing device 162, such as anAC tachometer. The gearing is correlated so as to be proportional to thelinear rate at which the follower device 142 moves the indicator device146 and hence, the drive element 159 in a forward direction. Preferably,the fit between the position indicator 146 and the rod 147 upon which itslides should be sufficiently tight so that when the carriage 71 isretracted, the device 146 will remain at the position to which it wasmoved by the follower device 142 prior to retraction thereof. By thisarrangement, the position indicator device 146 serves as a relativelyfixed memory means which will act to initiate the preimpact orintermediate reduced speed of the drill mechanism 22 at a distanceapproximately one-eighth of an inch from the undrilled material of theworkpiece W.

In the invention, means are provided for automatically determining thatthe drill bit 23 has completed drilling of a given hole to apredetermined depth. This may be accomplished by a depth-limiting device164 which may be preadjusted axially of the housing 67. Contact of theposition indicator 146 with the device 164 produces a signal whichindicates that the desired hole depth has been reached. The device 164may be slidably mounted on a pair of guide rods 165 (FIG. 9) which arefixedly mounted between the end walls 68 of the housing 67. A push rod166 is attached at one end to the device 164 and extends toward itsother end through a bushing 168 provided in the forward wall 68 of thehousing, A knob 169 may be provided on the end of the rod to provide amanual actuation therefor. Suitable indicia (not shown) may be disposedalong the rod for predetermining the desired hole depth.

In the form shown, the depth-limiting device 164 may in- (FIG. 8) havinga fixed contact 171 and a movable contact 172 which may be connected byconductors (not shown) to the control system for indicating that a holehas been drilled to the desired depth. The contact 172 may be in theform of a leaf spring which is laterally disposed relative to theposition indicator 146, whereby when the depth-limiting device 146 isdisposed generally. opposite the device 164, a projecting camlikeelement 173 on the device 146 will close the contacts 171 and 172 of thedevice 164.

In the invention, means may be provided for automatically powertransformer PT2 receive in interlocking relation therein a projection190 on the follower device 142. The latch plate 186 may be spring-biasedto hold the same in the nonlocking position, as seen in FIG. 8. Aspring-loaded overcenter snap device 163 is pivotally mounted, as at192, on the body 193 of the device 164 including projections 167 so thatthe latch plate 186 may be pivoted, such as in a counterclockwisedirection for interlocking engagement with the projection 190 uponengagement with the projections 167. By this arrangement, uponretraction of the carriage 71 and hence, the drill mechanism 22, theposition indicator device 146 is moved, as a unit, with the followerdevice 142 to the initial starting position for the next drillingoperation.

An unlatching device 175 in the form of a resilient generally V-shapedhook member 176 may be provided to uncouple the follower device 142 fromthe position indicator device 146. The hook member 176 may be mounted onone of the end walls 68 of the housing (FIG. 8) and in the path oftravel of the device 146 so that it may engage and strip the latch plate186 in a clockwise direction from its interlocking engagement with theprojection 190 on the follower device 142.

In the invention, it is to be understood that more than one drill headassembly 20 may be employed for independent or simultaneous drillingoperations with respect to the workpiece. For example, a pair of drillhead assemblies may be mounted on opposed sides of the support column 16for independent or simultaneous drilling operations. Accordingly, it iscontemplated that any number of such assemblies may be employed in theinvention, as desired.

ELECTRICAL CIRCUIT Referring now to FIGS. 13 through 19, there isillustrated electrical circuitry adapted for varying the operation ofthe mechanism 86, and more specifically, for controlling the flow ofcurrent through the servo valve solenoid coil 90.

The electrical circuitry includes a main power circuit A (FIG. 16), aprimary control circuit B (FIG. 13) adapted for selectively controllingthe flow of current through the servo valve solenoid coil 90, and asecondary control circuit C (FIG. 14) which is operably connected to theprimary control circuit to provide electrical signals for selectiveenergization of the components therein, thus control the rate anddirection of axial movement of the carriage 71.

Referring now to FIG. 15, the primary power circuit includes terminalsT1 and T2 which may be connected across a suitable source of electricalpower, such as 440 volts AC. The terminals T1 and T2 may be connected tothe primary P of a suitable stepdown power transformer TT1 by means ofconductors 210 and 211, respectively. The secondary S of the may beconnected to the primary control circuit B and secondary control circuitC through suitable rectifying means, illustrated generally at 212 forsupplying controlled DC power to such circuits. The primary controlcircuit may be connected to the secondary of the power transformer PT 2by means of conductors 201, 207 and 200 while the secondary controlcircuit C may be connected thereto by conductors 208, 209 and 201 withconductor 201 being common to both the primary control circuit B and thesecondary control circuit C.

Referring now to FIG. 13, the primary control circuit Bis l adapted toselectively vary the direction and magnitude of returning the positionindicator device 146 from its relatively A set position, as determinedby the depth-limiting device 164, for drilling the next successive holein the workpiece W. This may be accomplished by tapered camlikeprojections 163 on the device 164 (FIG. 8) which coact with a latchmechanism 185 on the device 146. The mechanism 185 includes a latchplate 186 pivotally mounted, as at 187, on the body 168 of the device146 and includes a notch portion .189 adapted to current flow throughthe servo valve solenoid coil 90, and includes a plurality of branchcircuits designated generally at the right-hand side of FIG. 13. Asshown, the primary control circuit B includes a forward rapid-feedcircuit 202, a forward intermediate-feed circuit 203, a cutting feedcircuit 204, and a retract circuit 206. The cutting feed circuit 204 maybe provided with a bypass circuit 205 which selectively interacts withthe intermediate-feed circuit 203 to provide a variable lowcurrent flowto the servo solenoid coil 90. By the selective actuation of suchcircuits, such as by control relays or the like (designated generally asCR), the magnitude and polarity of the magnetizing current to thesolenoid coil 90 may be controlled to move the spool valve and thus,regulate the rate and direction of flow to the motor.

In discussing the primary control circuit B, selective reference will bemade to various control relays. The relay coils of each control relaywill be designated by a numerical suffix, such as CR1, CR2, etc., whilethe various relay contacts of each control relay will be designated byan additional letter suffix, such as CRlA, CRIB, etc.

The forward rapid-feed circuit 202 is provided to move the carriage 71forwardly at a maximum feed rate. The forward rapid-feed circuit 202preferably includes a conductor 213, normally open contact CR2A,conductor 214, resistor 215, conductor 216, normally closed contactCR3A, conductor 217, servo solenoid coil 90, conductor 218, normallyopen contact CR2B and conductor 219, which connect the servo solenoidcoil 90 across the power conductors 200 and 201 when the contacts CR2A,CR3A and CR2B are closed. The total resistance of the forward rapid-feedcircuit is sufficiently low, such as 2.5 kiloohms, resulting in a highcurrent flow in a forward direction through the servo solenoid coil 90.By this arrangement, the spool valve will be moved to the full forwardflow position so that fluid flows into the fluid motor 81 at a maximumrate to move the carriage 71 forward at maximum speed, such as 500inches per minute.

The forward intermediate-feed circuit 203 is provided to feed thecarriage 71 forwardly at a preimpact rate which is relatively less thanthe forward rapid rate. As shown, the forward intermediate-feed circuit203 includes conductor 213, normally open contact CR2A, conductor 214,resistor 215, conductor 216 as far as the junction 221, conductor 222,resistor 223, variable resistor 224, conductor 225, servo solenoid coil90, conductor 218, normally open contact CR2B, and conductor 219, whichconnect the servo solenoid coil 90 across the power conductors 200 and201 when contacts CR2A and CR2B are closed and contact Cr3A is open. Theeffective resistance of this circuit is sufficiently high to cause arelatively low current flow through the servo solenoid coil 90. Forexample, the resistors 224, 223 and 215 may have resistance values of5,l2 and 2.5 kiloohms, respectively, to provide a total resistance of 19.5kiloohms. The variable resistor 224 is selectively adjustable to providecontrol of the magnitude of the current in the intermediate-feedcircuit, and thus, enable the preimpact feed rate to be preset at apredetermined value. This arrangement will result in a relatively lowcurrent flow in a forward direction through the servo solenoid coil 90causing the spool valve to be biased in a partially open position, andthus, enable fluid to flow into the fluid motor 81 at a relatively slowrate for moving the carriage 71 forward at a slower preimpact rate ofinches per minute.

The cutting feed circuit 204, when switched into operation, continuallyvaries the current flow through the servo solenoid coil 90, in responseto feedback signals from the secondary control circuit C, whereby theforward movement of the carriage is maintained at a relatively constantslowest speed during cutting engagement of the drill with the metal. Thecutting feed circuit 204 includes the intermediate forward feed circuit203 above described, and a bypass circuit 205 which is adapted toprovide a shunting circuit around the servo solenoid coil 90. The bypasscircuit includes conductor 231, contact CR3B, conductor 232,silicon-controlled rectifier 230 (hereinafter designated as SCR 230) andconductor 233, which are connected between the power conductor 201 and apoint 234 on the conductor 216 between the contacts CR3A and thejuncture 221, so that the flow of current between the conductors 200 and201 in the intermediate forward feed circuit 203 is diverted or shuntedaround the servo coil 90 when such shunt circuit is switched intooperation. Such switching operation occurs when contact CR3B is closedand the SCR 230 is firing. When the bypass circuit 205 is in operation,the timing is such that the magnitude of the average current flowthrough the servo solenoid coil 90 will be less than the magnitude ofthe current which had been flowing in the servo solenoid coil when onlythe intermediate circuit was in operation. In accordance with theinvention, the net low current will be a variable having a magnitudeduring any given time period depending upon the timing and duration ofthe firing of the SCR 230 which, in turn, is controlled by feedbacknetwork 320, which will be discussed in more detail hereinafter. Sucharrangement results in' a continual hunting" action causing rapidhunting oscillation of the spool valve to maintain a relatively constantslow rate of flow of fluid into the fluid motor 81, and thus, enable thedrill bit 23 to be fed forwardly at a relatively constant slow rateduring the cutting operation.

The retract circuit 206 operates to cause the carriage to be retractedfrom the workpiece. The retract circuit 206 includes conductor 241,normally closed contact CR2C, conductor 242, resistor 243, conductor244, servo solenoid coil 90, conductor 245, normally closed contact CR2Dand conductor 246 which connect the servo solenoid coil across theconductors 200 and 201 in a reverse direction when contacts CR2C andCR2D are closed and contact CR3A of the forward rapid-feed circuit 202is open. The resistance of the retract circuit 206 is sufiiciently low,such as 3 kiloohms, so that a high current flows through the servosolenoid coil 90. This high reverse current flow through the servosolenoid coil 90 causes the spool valve to move to thefull reverse flowposition. With the spool valve in this position, fluid will flow intothe fluid motor 81 in the reverse direction causing the carriage toretract at the same rate as the maximum forward rapid-feed rate, such as500 inches per minute.

A cycle maintenance circuit 250 may be provided for establishing andmaintaining the cycling operation throughout the drilling of each hole.The cycle maintenance circuit 251 includes relay coil CR1, conductor252, reject relay 253, conductors 265 and 273, normally open contactCRlA, conductor 272, depth limit switch 21A and conductor 271. The cyclemaintenance circuit 250 is energized by energization of either themanual start circuit 258 or the tape start circuit 259 which causesenergization of relay coil CR1. Energization of relay coil CR1 closesnormal open contact CRlA completing the circuit between conductors 207and 201 when the limit switch 21A is closed. The cycle maintenancecircuit will remain energized throughout the drilling operation untilthe limit switch 21A is opened upon completion of a hole causingdeenergization of relay coil CR1 and opening of relay contact CRlA.

The manual start circuit 258 includes conductor 251, relay coil CR1,conductor 252, normally closed manual reject relay 253, conductors 265and 254, manual start pushbutton 255 and conductor 256. Such circuitarrangement momentarily connects relay coil CR1 across the conductor 207and 201 when the manual relay switch 253 is closed, and the manual startpushbutton 255 has been momentarily depressed. Similarly, tape actuationof the cycle maintenance circuit 250 arises from the fact that relaycoil CR1 is connected in the tape start circuit 259. The tape startcircuit 259 includes conductor 261, normally open tape switch 262,conductors 263, 264 and 265, normally closed manual reject switch 253,conductor 252 and conductor 251. By such arrangement, relay coil CR1 isconnected between the higher potential conductor 207 and lower potentialconductor, as at 267, to cause energization of relay coil CR1 whenreject switch 253 is closed and the tape start switch 262 is momentarilyclosed.

A rapid-feed triggering circuit 280 is provided to control energizationof the forward rapid-feed circuit 202. The rapidfeed triggering circuit280 includes conductor 275, limit switch 14a, conductor 276, contactCRIB, conductor 277 to juncture 285, conductor 281, contact CR4A,conductor 282, relay coil CR2, and conductor 283., By such circuitry,relay coil CR2 is connected across the conductors 200 and 207 andenergized upon the closing of limit switch 14a, contact CRIB contactCR4A. Energization of relay coil CR2 closes the contacts CR2A and CR2Bin the forward rapid-feed circuit 202 thereby applying a maximum currentto the servo solenoid coil 90, as aforesaid. A safety feature isprovided in that limit switch 14a can be closed only when the carriageis in the fully retracted position. As a result, forward movement of thecarriage 71 cannot commence until the carriage is completely retractedto close the limit switch 140.

The rapid-feed holding circuit 290 is provided to sustain forwardmovement of the carriage 71 until retraction of the same is required.The rapid-feed holding circuit 290 comprises a main circuit includingconductor 283, relay coil CR2, conductor 282, normally closed contactCR4A, conductors 281 and 278, contact CRZE, conductors 284, 264 and 273,contact CRlA, conductor 272, limit switch 21A and conduc tor 271. Bysuch circuitry, relay coil CR2 is connected across conductor 201 and207, and is energized when limit switch 21a and contact CRlA, CRZE andCR4A are closed. Since normally open contact CR2E will not be closeduntil relay coil CR2 is energized, the rapid-feed holding circuit isinterconnected to the rapid-feed triggering circuit 280 at the juncture285. Upon energization of the rapid-feed triggering circuit, all of theaforementioned contacts are closed with the exception of contact CRZE.Therefore, the rapid-feed holding circuit 290 will continue to beenergized until a retract signal is received upon opening of one of theswitches of contacts therein. As can be seen, this portion of therapid-feed triggering circuit including conductor 275, limit switch 14a,conductor 276, normally open contact CRIB and conductor 277 forms abranch circuit for the rapid-feed holding circuit. By this arrangement,the limit switch 14a and relay contact CRIB will be closed when thecarriage 71 is momentarily in the fully retracted position therebycausing energization of the relay coil CR2, and thus, close contactCR2E. As the rapid-feed holding circuit 290 is simultaneously energized,when the carriage 71 is in the fully retracted position, the coil CR2will remain energized through the contact CR2E even though limit switch14a will open as the carriage 71 moves forwardly.

The intermediate-feed triggering circuit 295 controls the energizationof the intermediate forward feed circuit 203. The intermediate-feedtriggering circuit 295 includes conductor 296, relay coil CR3, conductor297, limit switch a and conductor 298 which connect relay coil CR3across the conductors 201 and 207 when the limit switch 15a is closed.The limit switch 15a is closed when the carriage follower finger 144makes physical contact with the limit switch 15a about oneeighth of aninch before contact of the drill bit 23 with the undrilled metal.Energization of the relay coil CR3 opens a normally closed contact CR3Ain the forward rapid-feed circuit 202 thereby directing the current flowto the intermediatefeed circuit 203 to cause the carriage to decelerateto a relatively slow intermediate speed.

A retract triggering circuit 302 is provided to control the energizationof the retract circuit 206 and may include conductor 303,silicon-controlled rectifier 304 (hereinafter designated SCR304),conductor 305, resistor 306, relay coil comparison circuit 332 whichprovides an error signal for intermittently firing the SCR230 (FIGS. 14and 15) to shunt CR4 and conductors 307 which connect the relay coil CR42 between the conductor 201 and 200. Conductor 308 connects the gate ofthe SCR304 to the secondary control circuit C and is adapted to transmita signal for firing the SCR304. When the SCR304 fires, the relay coilCR4 will be energized opening contact CR4A in the rapid-feed holdingcircuit 290 thereby deenergizing relay coil CR2. The deenergization ofrelay coil CR2 opens contacts CR2A and CRZB in the forward rapidfeedcircuit 202 and closed contacts CR2C and CR2D in the retract circuit 206causing retraction of the carriage in the manner previously described.

Referring now to FIG. 14, the secondary control circuit C is provided tosense the axial and rotational movements of the drill bit 23 and providesignals for automatically controlling the respective branch circuits ofthe primary control circuit and to control the movements of the drillbit during cutting engagement with the workpiece. As shown, thesecondary control circuit C includes a sensing device 54, such as a DCtachometer, which senses the rate of axial movement of the drill 23shortly after contact of the drill bit 23 with the metal and a feedbackcircuit 320 which receives the output of the DC tachometer. Preferably,the feedback circuit includes a current from the intermediate feedcircuit 203 in the manner previously described. By this arrangement, thecurrent flows to the servo solenoid coil is reduced to achieve thedesired slow speed. Such intermittent reductions in current flow occurmore rapidly than the spool valve can oscillate, thereby resulting in aconstant slow speed.

The DC tachometer 162 is driven by carriage follower through chain 159for sensing the carriage forward speed. Since the voltage output of theDC-tachometer 162 is proportional to the axialspeed of the carriage, itprovides the input for the feedback circuit 320. Such input may be fed,for example, into the voltage comparison circuit 322 wherein the outputof the DC tachometer is compared to a reference value. The referencevalue may be preset at a voltage output of the DC tachometer whichcorresponds to a given rate of travel of the carriage 71, such as 6inches per minute. The voltage comparison circuit 322 may be anysuitable circuit known in the art, but preferably it is one which,through a suitable amplifier 323 would produce a positive output signalwhenever the DC tachometer voltage exceeds the reference value, with themagnitude of the error signal being proportional to the value by whichthe reference value is exceeded. This output signal may then be fed to asuitable pulse-generating system, as known in the art, such as at 324,which may include a transistor 325, capacitor 326, unijunctiontransistor 327 and load resistance 328. The load resistance 328 isdisposed in the gate circuit of the SCR230, and the SCR230 and thepulse-generating system 324 may be connected to the same rectifiednonfiltered power source (FlG. 15). By such an arrangement, when theforward speed of the carriage 71 exceeds the desired rate of 6 inchesper minute, the variations in the magnitude of the error signal willcause corresponding variations in the length of the firing time of theSCR230. The longer the SCR230 fires in each voltage cycle, the morecurrent is shunted from the servo valve solenoid coil 90 through thebypass circuit 205, and consequently, the spool valve moved to a moreclosed position. Such action causes the consequent reduction in theforward speed of the carriage 71 and a corresponding reduction in theoutput of the DC tachometer 162. When the DC output of the tachometerdrops below the reference value, SCR230 is turned off and the fullintermediate current flows through the solenoid coil 90 tending to raisethe axial speed of the carriage 11. When the output again exceeds thereference; value, SCR230 again fires causing the speed to be reduced. Bythis closed loop hunting action, the desired accurate constant slowaxial forward speed of the drill is achieved. Once the speed has beenreduced to the desired 6 inches per minute, it remains effectivelyconstant during the entire cutting action.

The secondary control circuit C may also be provided with a chip buildupindicating circuit 330 which is provided to operably connect the servosolenoid coil 90 in the retract circuit 206 when the speed of rotationof the drill bit 23 drops below a predetermined value. In the preferredform, an AC tachometer 54 may be provided to sense the rotational speedof the drill bit 23, and the output of the AC tachometer 162 may be fedto a threshold level sensing circuit 300. [n the form shown, thethreshold level sensing circuit 300 includes a transistor 29] having itscollector terminal connected by conductor 308 to the gate of the SCR304in the retract triggering circuit 302 (FIG. 13). The SCR304 is kept fromfiring as long as the output of the AC tachometer is above the thresholdlevel and the gate current is below the minimum required value forfiring. When the output of the AC tachometer falls below the thresholdlevel, the gate current will be increased sufficiently to cause theSCR304 to fire thereby energizing relay coil CR4 and causing the retractcircuit 206 to be energized.

Retraction of the carriage 71 upon completion of the drilling of a holeis initiated by the interlocking of position indicator 146 and depthlimit device 164 (FIG. 8) causing the opening of limit switch 210 in thecycle maintenance circuit 250 (FIG. 14), and the consequentlydeenergization of relay coils CR1 and CR2. The deenergization of suchrelay coils opens contact CR2A in the forward rapid-feed circuit 202 andallows contact CRZC and CR2D in the retract circuit 206 to return totheir normally closed positions thereby connecting the servo solenoidcoil 90 in the retract circuit 206 to cause high-speed retraction of thedrill carriage.

A hole completion signalling circuit 310 is provided to indicate that ahole has been completed. The hole completion signalling circuit 310includes conductors 311, capacitor 312, conductor 313, contact CRlC,conductor 314, contact CR3C, and conductor 315. The hole completionsignalling circuit 312 also includes conductors 316, contact CR3D,resistor 317 and conductor 318. The branch conductor 311 connects oneside of the hole completion signalling circuit 310 to the conductor 201while the conductor 318 connects the branch circuit to the conductor200. The branch circuit is interconnected to the conductor 314 by theconductor 316 whereby the capacitor 312 will be charged upon closing ofthe contacts CRlC and CR3D. The indication that a hole has beencompleted is initiated by the opening of limit switch A when thefollower finger 144 is disengaged from the limit switch 15A by thestripping hook 176 which occurs when the carriage is in a full retractposition. The opening of switch 15a deenergizes relay coil CR3 incircuit 295 thereby allowing contact CR3C in the hole completionsignalling circuit 310 to return to its normally closed position andcontact CR3D to return to its normally open position enabling thecapacitor 312 to discharge along the conductor 313 which is connected toa signal-sensing device (not shown). Where the machine is adapted fortape initiation of rotation of the worktable 14 for drilling of the nexthole, such signal provides the signal to the tape reader to commence theindexing. Since the hole-signalling circuit 310 cannot operate until thecarriage 71 has returned to the full retract position to open limitswitch 14a, the indexing cannot prematurely begin while the drill bit 23is in motion or in contact with the work, thus providing a safetyfeature. In addition, since the relay coil CR1 in the cycle maintenancecircuit 250 is deenergized, the carriage 71 will not move forwarddespite the contact of the cam of the carriage 71 closing limit switch14a as the forward motion must await the act of deenergization of relaycoil CR1 either by tape signal or manual start.

The circuitry for the tape device may be any conventional circuitrywhich, upon receipt of a signal from the hole completion signallingcircuit 310 will generate control signals to cause the motor 23 torotate the worktable 14 the'desired number of preprogrammed degrees forthe commencement of the next hole. When the table has been rotated tothe proper position, the programmed tape should then transmit a startsignal in the tape start circuit 267 to reenergize the relay coil CR1and recommence the forward rapid-feed of the carriage 71 in therecycling operation, as aforesaid.

Referring again to FIG. 15, the main power circuit A may includeoperating coils 320 and 321 for starting a hydraulic motor and foroperating the drill motor at idling speed, respectively. A high-speedoperating coil 335 may be connected across the conductors 210 and 211when the contact CRlD is closed. Since contact CRlD is closed wheneverthe contact relay coil CR1 and a cycle maintenance circuit is closed,the high-speed motor is on whenever the cycling operation is inprogress, and the motor returned to idling speed while the table isindexed for the drilling of the next hole. The coils 320 and 321 will beconnected across the terminals T1 and T2 through conductors 337 and 211upon depressing of the manual start button 322 while coil 355 requiresthe contact CRlD to be closed. In addition, the main power circuit mayalso include a coolant valve actuating circuit 340 which includesconductors 351, coolant solenoid coil 356, conductor 342, normallyclosed contact CR3F, conductor 343, 344 and 345, normally open contactCRlD, and conductor 337, which connect the coolant valve solenoid coil356 across the power conductors 210 and 211. The coil 356 is energizedwhen the contacts CRlD and CR3F are closed during the high-speedOPERATION For a typical operation of the drilling apparatus 2, referencemay be had to FIGS. 18 and 19 which diagrammatically illustrate thesequence of steps for the incremental drilling of a hole to apredetermined depth in the workpiece W. The workpiece may be mounted onthe rotatable support table 14 and the drill head assembly 20 loweredvia the column 16 so that the drill bit 23 may be angularly oriented byadjustment of the bracket assembly 18 with respect to the workpiece. Theassembly 20 is lowered into the workpiece (i.e., cavity) until theposition indicator device 146 is disposed adjacent the area for drillinga hole and the depth limit device 164 (FIG. 8) is adjusted by actuationof the control knob 169. In the case of a fully automatic controloperation, a programmed tape having the desired hole pattern and depthmay be employed with the apparatus. The operation is commenced bypressing a pushbutton 322 (FIG. 15) which actuates the fluid supplysource, such as a hydraulic pump (not shown), so that the drill bit 23commences rotation and for energizing the power for the variouscomponents of the electrical circuit. At such time, the carriage 71 willbe in the full retracted position (FIG. 5) or will automatically takesuch position. A start pushbutton 255 may then be actuated which willenergize the cycle maintenance circuit 250 (FIG. 13) which rotates thedrill bit 23 at a high speed, such as 10,000 rpm, and which actuates theservicing device 130 for delivering coolant and/or lubricant to the bit23. Since at the start of each cycle the drill mechanism 22 is fullyretracted, the cam 139 (FIG. 9) acts to hold switch 140 closed, theholding circuit 290, and the rapid-feed triggering 280. The rapid-feedcircuit 202 provides a high current for the solenoid coil to open theservomechanism 86 to actuate the fluid motor unit 81 to move the drillmechanism 22 toward the workpiece W at a maximum feed rate, such as 500inches per minute, as seen at A in FIG. 20. During this high-speedmovement, coolant and/or lubricant is discharged onto the drill bit 23to lubricate the same and remove any chips therefrom.

The holding circuit 290 maintains the rapid forward motion of the drillmechanism 22 even though the switch 14a opens upon such forwardmovement. This circuit also maintains forward motion of the drillmechanism 22 during the two subsequent forward speeds and upon beingdeenergized, for any reason, enables the drill mechanism 22 to beautomatically retracted.

As best seen at B of FIG. 19, when the drill bit 23 is approximatelyone-eighth of an inch from the workpiece W, the intermediate switch 15ais actuated by the finger 144 of the follower device 142 (FIG. 8) whichenergizes the forward intermediate-feed circuit 203 (FIG. 13) reducingthe current in the solenoid coil 90 which causes the spool valve topartially close to reduce the flow to the motor unit 81. This slows thetravel of the drill mechanism 22 to a preimpact or intermediate rate,such as 10 inches per minute. Energization of the feed circuit 203 alsooperates to deactivate the servicing unit for feeding coolant and/orlubricant.

After the drill bit 23 has entered the workpiece, as at C of FIG. 19,the sensing device 162 produces an electrical signal proportional to theaxial rate of movement of the drill mechanism 22. This output is fed toa voltage comparison circuit 322 where it is compared to a standard orreference voltage representative of what the voltage output of thedevice 162 would be if the forward speed of the drill carriage 71 were 6inches per minute, for example. Whenever the input voltage is above thereference value, an error signal is produced which is amplified, andthen converted to a pulse signal, such as by the generator 324 to firethe SCR230. Firing the SCR230 closes the current bypass circuit 205which reduces the cur-

1. A drilling apparatus for drilling one or more holes in a workpiececomprising, a frame, a drill mechanism including a drill bit supportedon said frame for movement relative to said workpiece, drive means forrotating and for moving said drill bit axially toward and away from saidworkpiece, control means operably associated with said drive means formoving said drill bit at least at one predetermined rapid-feed rateprior to engagement with said workpiece and at a predetermined,relatively slower, substantially constant cutting feed rate duringengagement with said workpiece, sensing means for determining the axialfeed rate of said drill bit during cutting coacting engagement with saidworkpiece, feedback means operably associated with said sensing meansbeing adapted to produce an output signal related to said predeterminedcutting feed rate and the actual feed rate of said drill bit during saidcutting coacting engagement with said workpiece, and said control meansarranged to actuate said drive means in response to said outpuT signalof said feedback means to move and maintain said drill bit at saidpredetermined, substantially constant, cutting feed rate.
 2. In adrilling apparatus in accordance with claim 1, wherein said drive meansincludes, a fluid pressure motive means adapted for moving said drillbit with more than one predetermined axial feed rate toward and incutting coaction with said workpiece and for retracting said bit at apredetermined rate away from said workpiece.
 3. In a drilling apparatusin accordance with claim 2, wherein said motive means includes, areciprocal fluid pressure motor unit, and control valve means forregulating the flow of fluid pressure to said motor unit from a supplysource.
 4. In a drilling apparatus in accordance with claim 3, includingelectrical control means operably associated with said control valvemeans for controlling reciprocal movement of said motor unit in responseto the rate of rotation of said drill bit.
 5. In a drilling apparatus inaccordance with claim 1, wherein said control means includes, firstswitch means operably associated with said drive means for moving saiddrill bit axially toward said workpiece at a relatively highpredetermined rate of speed.
 6. In a drilling apparatus in accordancewith claim 5, wherein said control means includes, second switch meansoperably associated with said drive means for automatically reducingsaid predetermined speed rate to a relatively lower predetermined rateof axial speed and at a predetermined distance from said workpiece. 7.In a drilling apparatus in accordance with claim 1, including depthcontrol means operably associated with said drive means forautomatically retracting said drill bit from cutting coaction with saidworkpiece after forward movement of said bit to a predetermined depth insaid workpiece.
 8. In a drilling apparatus in accordance with claim 1,including service means operably associated with said drill mechanismfor automatically delivering a cooling and/or lubricating media to saiddrill bit in response to movement thereof toward and away from saidworkpiece.
 9. A control system including a servomechanism forcontrolling movement of a drill mechanism with respect to a workpiececomprising, a primary control circuit adapted to be connected to asource of electrical power, said servomechanism electrically connectedwith said primary control circuit for actuating a drill mechanism, saidprimary control circuit including a first circuit electrically connectedto said servomechanism for moving said drill mechanism in a forwarddirection at one feed rate, a second circuit electrically connected tosaid servomechanism for moving said drill mechanism in a forwarddirection at another feed rate, a first switching means operablyconnected to said drill mechanism, said switching means sequentiallyelectrically connecting said first and second circuits to saidservomechanism, a third circuit electrically connected to saidservomechanism, a second switching means electrically connected to saidthird circuit to energize said third circuit for moving said drillmechanism at still another forward feed rate, feedback means operablyconnected to said drill mechanism and electrically connected to saidthird circuit and said switching means, and said feedback means beingresponsive to the feed rate of said drill mechanism to automaticallyactuate said switching means and to energize said third circuit tomaintain a relatively constant forward feed rate of said drillmechanism.
 10. A control system including a servomechanism forcontrolling movement of a drill mechanism with respect to a workpiececomprising, a primary control circuit adapted to be connected to asource of electrical power, said servomechanism electrically connectedwith said primary control circuit for actuating a drill mechanism, saidprimary control circuit including a first ciRcuit electrically connectedto said servomechanism for moving said drill mechanism in a forwarddirection at one feed rate, a second circuit electrically connected tosaid servomechanism for moving said drill mechanism in a forwarddirection at another feed rate, a first switching means operablyconnected to said drill mechanism, said switching means sequentiallyelectrically connecting said first and second circuits to saidservomechanism, a third circuit electrically connected to saidservomechanism, a second switching means electrically connected to saidthird circuit to energize said third circuit for moving said drillmechanism at still another forward feed rate, a fourth circuit operablyconnected to said first, second and third circuits, and electricallyconnected to said switching means for predetermined deenergization ofsaid first, second and third circuits, and retract signal meanselectrically connected with said fourth circuit to provide a signal toenergize said fourth circuit when the rotational speed of said drillmechanism changes relative to a predetermined speed.
 11. A controlsystem including a servomechanism for controlling movement of a drillmechanism with respect to a workpiece comprising, a primary controlcircuit adapted to be connected to a source of electrical power, saidservomechanism electrically connected with said primary control circuitfor actuating a drill mechanism, said primary control circuit including,a first circuit electrically connected to said servomechanism for movingsaid drill mechanism in a forward direction at one feed rate, a secondcircuit electrically connected to said servomechanism for moving saiddrill mechanism in a forward direction at another feed rate, a firstswitching means operably connected to said drill mechanism, saidswitching means sequentially electrically connecting said first andsecond circuits to said servomechanism, and speed-responsive retractionmeans operably connected to said drill mechanism and electricallyconnected to said primary circuit to move said drill mechanism away froma workpiece.
 12. A drilling apparatus in accordance with claim 1,including another sensing means operably associated with said drivemeans for retracting said drill bit from cutting coaction with saidworkpiece in response to reduction in the rate of rotation of said drillbit.
 13. A drilling apparatus in accordance with claim 1, wherein saidsensing means comprises a tachometer generator operably mounted on saiddrill mechanism for producing an output signal proportional to the axialspeed of said drill bit.
 14. A drilling apparatus in accordance withclaim 12, wherein said another sensing means comprises a tachometergenerator operably connected to said drill bit adapted to produce anoutput signal proportional to the rotational speed of said drill bit.15. A drilling apparatus in accordance with claim 1, includingpressure-responsive sensing means operably connected to said drill bitfor sensing the axial pressure on said drill bit during cutting coactionwith said workpiece, and said pressure-responsive sensing meansincluding a switching device arranged to be actuated upon application ofan axial pressure on said drill bit in excess of a predeterminedpressure to retract said drill bit from said workpiece.
 16. A drillingapparatus in accordance with claim 1, wherein said drill mechanism ispivotally mounted on said frame for pivotal movement of said drill bitabout a generally horizontal axis.
 17. A drilling apparatus inaccordance with claim 1, wherein said control means include aservomechanism for controlling the movement of said drill mechanism withrespect to said workpiece, a primary control circuit adapted to beconnected to a source of electrical power, said servomechanismelectrically connected with said primary control circuit for actuatingsaid drill mechanism, said primary control circuit including a firstcircuit electrically connected to said servomechanism for moving saiddrill mechanism in a forward direction at one feed rate, a secondcircuit electrically connected to said servomechanism for moving saiddrill mechanism in a forward direction at another feed rate relativelyslower than said one feed rate, a first switching means operablyconnected to said drill mechanism, and said switching means sequentiallyelectrically connecting said first and second circuits to saidservomechanism.
 18. A drilling apparatus in accordance with claim 17,wherein said servomechanism includes a current-responsive servo member,said servo member electrically connected to said primary controlcircuit, and said second circuit having a greater resistance than saidfirst circuit.
 19. A drilling apparatus in accordance with claim 17,including a third circuit electrically connected to said servomechanism,and a second switching means electrically connected to said thirdcircuit to energize said third circuit for moving said drill mechanismforwardly at said predetermined cutting feed rate upon cutting coactingengagement of said drill bit with said workpiece.
 20. A drillingapparatus in accordance with claim 19, wherein said feedback means isoperably connected to said drill mechanism and electrically connected tosaid third circuit and said switching means, and said feedback meansbeing responsive to automatically actuate said switching means and toenergize said third circuit to maintain a relatively constant forwardfeed rate during cutting coaction of said drill bit with said workpiece.21. A drilling apparatus in accordance with claim 19, including a fourthcircuit operably connected to said first, second and third circuits, andelectrically connected to said switching means for predetermineddeenergization of said first, second and third circuits.
 22. A drillingapparatus in accordance with claim 21, wherein said another sensingmeans being electrically connected with said fourth circuit to provide asignal output to energize said fourth circuit when the rotational speedof said drill mechanism changes relative to a predetermined speed.
 23. Adrilling apparatus in accordance with claim 1, including a supportcolumn extending from said frame, at least one drill head assemblydepending from said support column for pivotal movement about agenerally horizontal axis, said drill head assembly including a drillcontrol mechanism, and said drill mechanism operably mounted on saiddrill head assembly being disposed for axial movement relative to saiddrill control mechanism toward and away from said workpiece.
 24. Adrilling apparatus in accordance with claim 23, including means forrotating said drill head assembly about a generally vertical axis, andmeans for rotating said workpiece on said frame.
 25. A drillingapparatus in accordance with claim 23, including a pair of said drillhead assemblies mounted on and depending from opposed sides of saidsupport column each including a drill mechanism for independent orsimultaneous drilling operation with respect to said workpiece orworkpieces.